Parking Garage Slab Uplift From Water Table

[cvanoverbeke]

I understand the physics of concrete slab uplift from installing it below the water table. It is the rho g h concept. In the case we are looking at the geotechnical report indicates that the parking garage should be installed at 321.7 metres above sea level. The water table is 0.5 metres below this level there 321.2 m ASL. The top of the parking garage floor is at 317.5 m ASL. Preliminary calculations give a load of over 1000 lb/sq.ft. This is quite high.

Can anyone provide example calculations for the uplift pressue on this slab?

Can anyone provide examples of alternative construction technologies besides making the slab 3 to 4 feet thick to resist the uplift pressue. We have considered permannet dewatering but the risk of not getting regulatory approval is too high to consider this option.

Regards,
Cvanoverbeke

 

[dik]

Assuming good drainage, the upward pressure will be equivalent to the head of water above the U/S of the slab. Depending on the drainage the pressure will be diminished.

Usually construct 'pop-outs'... These are loose panels in the slab-on-grade that are intended to fail by lifting when the water pressure is sufficient.

Dik

 

[msquared48]

If this is a seasonal or storm driven scenario and not a permanent one, why not install a sump pump system hooked to the foundation drain to lower the water table in the vicinity of the exterior walls. If the water pressure is lower at the walls, if will also be so at the floor for uplift. You will probably need backup generators if the power fails too.

Obviously, this is not a cool idea if the high water table is year around.

Mike McCann
MMC Engineering

 

[cvanoverbeke]

In response to Mike McCann's suggestions, the water table is permanent. Also, the sugggestion that you have mentioned is called permanent dewatering. Yes, a backup generator would be required but the biggest issue is obtaining the necesary permits (permit to take water from Ontario Ministry of Enviroment and permit to dump ground water in the storm sewer system from the local city)

I think we are just going to build on big thick piece of concrete. We will provide as much drainage as possible to reduce uplift pressures but you cannot rely on this from a permanent strength solution point of view.

Cvanoverbeke

 

[dicksewerrat]

How will you keep the water out of the garage? you are putting that flor over 12 feet below the water table. that calls for 'permanent dewatering'. Raise your bldg. Do you have a permit to build his thing that low?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[civeng80]

I use to design sewarage pumping stations and we had this problem most of the time.

The base would have to be quite thick to take the pressure and the whole building would have to be designed as a water retaining structure. Good quality Water stops would have to be installed between the walls and the base. A check would have to be done on boyancy uplift forces of the entire structure taking in account friction forces between the soil and the wall otherwise the entire structure may pop out of the ground if the groundwater rises far enough.

 

[SteveWag]

This type of construction is very common in waste water plants. When the structure is not heavy enough to withstand floating, mass is added, either by adding concrete or extending the slab to pick up earth load, or ground anchors are installed. If the structure is heavy enough, as in your situation (I think), the floor is designed as an upside down uniformly loaded slab spanning from column to column. If the ground water is above the lower level for more than a day or two, allowing ground water to enter the structure is not an option, and you will need good water stops at all joints as civeng80 has stated. There are several good books on the design of concrete tanks covering this information.
Steve Wagner

 

[civeng80]

I think one of your biggest problems is building the structure in those conditions, it is very difficult especially if you cannot dewater to a legal point of discharge.

 

[cvanoverbeke]

Response To Steve Wagnor

From a simple point of view, you are right. The building is like pushing a beach ball down under water. The buoyancy forces want to pop the building up. As for your comments on the building weighing enough, there is some validity to your comments. The whole building is 17.35 metres above the average grade and it is concrete construction. We have not calculated the weight of the building yet but I believe it will be heavy enough. I will look for concrete tank designs. I appreciate your feedback.

Response To civeng80

We can get a temporary dewatering permit. That appears to be a relatively easy permit to obtain. The problem is the permanent dewatering permit. The building is a 6 storey condominium therefore the developer does not wish to lose time taking up to 6 months to obtain a permanent dewatering permit that we may not get anyway. The geotechnical engineer suggested that the easiest construction solution is temporary dewatering with a thick slab. It really is a cost issue since you have to pour alot more concrete for this situation than a normal parking garage floor. I appreciate the feeback

Cvanoverbeke

 

[patswfc]

Yeah you dont have to make the base slab heavy enough to resist the flotation on its own, but if it isnt then it may float during construction until the building above has reached a point where its dead load including the dead load of the base slab can resist flotation.

To cater for this you can dewater during construction and include pressure release valves in the base that will allow water up through the base flooding the garage, if water pressures become too high in the event of something going wrong with the dewatering process. Once the weight of the structure is heavy enough to resist flotation you can stop dewatering.

One thing to be very careful of when dewatering is ground movements in the vacinity which may damage nearby structures (foundation settlement)

 

[ron9876]

We have this problem quite often with parking below sea level. It is normally cheaper to tie the slab down with piles. Especially if you are limited to 0.6D + H. You really chase your tail when you try to resist the uplift with weight of concrete since the water pressure increases with the depth of the slab. The piles have the added advantage of not requiring dewatering until enough building weight is in place which can be a substantial savings.

 

[oneintheeye]

dont think you want the dead weight of the slab only to resist the uplift or you'll be chasing your tail. Use the building weight. Water tight the structure. As someone said its like a sinple water tank in effect.